The Slitheen transporter pendant was a device that was used by the Slitheen for teleportation. It allowed them to teleport great distances. The Slitheen wore it as a necklace. They used it to escape from dangerous situations.
After Glune Fex Fize Sharlaveer-Slam Slitheen was killed with vinegar by Maria Jackson Kist Magg Thek Lutiven-Day Slitheen called his other family members and told them that they were attacked. His family members teleported to the school to help them. They used the Slitheen transporter pendant for teleportation. When the machine at the school was going to explode, most Slitheen family members used the the Slitheen transporter pendant to teleport away. (TV: Revenge of the Slitheen)
Korst Gogg Thek Lutiven-Day Slitheen wanted to use the tansporter pendant to teleport to his space ship, but the pendant was taken by Alan Jackson. Later Sarah Jane Smith used the pendant to teleport from the Pharos Institute to the attic of 13 Bannerman Road. She wanted to save Luke Smith from Mr Smith. (TV: The Lost Boy)